Voice coil motors (VCM) are commonly used for moving an arm that carries read/write heads over a spinning disk from a rest position on a parking ramp to a desired position, and vice-versa. Commonly, the disk is rotated by a brushless spindle motor such to position the heads over the tracks of a certain sector of the disk from which data is to be read or written.
Voice-coil motors are used in a number of applications. They are substantially composed of a winding immersed in a magnetic field generated by a permanent magnet. By forcing a certain current through the winding, the winding receives a displacing force. The displacement of the winding may be controlled with great precision.
In a disk storage device, it is important to control with high precision both the brushless spindle motor and the VCM. Modern fabrication technologies permit fabrication of integrated devices containing in a single package the power switches (bipolar junction or MOS transistors) of the output drivers of the brushless motor, and of the VCM for positioning the heads together with the control circuitries of both motors.
With these technologies allowing fabrication in a single chip through a single process logic CMOS devices, bipolar junction signal transistors, vertical or lateral power MOS or BJT devices, it has been possible to digitally implement many control functions. By using powerful CAD tools it has been possible to implement rather complex functions and controls of extremely high precision and performances in these integrated devices, commonly known as “power combo”.
Co-integrated control architectures based exclusively on digital techniques and circuits and output power stages make these devices adaptable to the user needs, and to different schemes of partitioning of the mass storage support.
The circuit for driving the brushless spindle motor for rotating the disk is generally composed of fully digital circuit blocks. Improvements toward a complete digitalization also of the control circuit of the VCM have been achieved, and they are leading to the realization of a control architecture of the VCM motor called “digital power processing voltage mode” or DPPV.
For example, FIGS. 1 and 2 depict respective integrated control systems of a brushless motor and of a VCM for positioning the heads on the rotating disk according to a modern embodiment on a single chip of a power combo device intended for a hard disk device (HDD).
The function of the different circuit blocks and of the main signals of the two control systems, used in various commercial devices of STMicroelectronics, depicted in FIGS. 1 and 2, are illustrated in TABLES 1 and 2:
TABLE 1Self Adapt. T.O.Circuit for rephrasing automatically the current in thespindle with its BEMF in order to optimize the motortorque.Curr. SignCircuit for determining the direction of the flow of thecurrent in the spindle.ADCAnalog-to-digital converter of the supply voltage andof other main voltages.Phase ShiftEnables the rephasing of the current of the spindle andof the relative BEMF upon a user input or upon asignal coming from the Self Adapt T.O.Feed-FwdCircuit for feed-forward compensating differencesbetween the supply voltage and the nominal value.Address generatorIt generates the address of the data stored in “Memory2 × 3 profile” corresponding to the instantaneousvalues of the voltage to be given to the phases of thespindle for generating an appropriate voltagewaveform.Memory 2 by 3Look up table that stores the data sequences thatprofilegenerate an appropriate voltage waveform (called2 × 3) to be supplied to the phases of the spindleduring its rotation.Digital multiplierA digital multiplier that allows modulation of theamplitude of each phase voltage of the spindle as afunction of the value coming from the speed controllerfor allowing the control of the speed.PWM ConverterIn case of three-phase motors, it converts three digitaldata with N bits in three phase modulated digital data(PSM).SPINDLE PSMPower stage for driving (in switched mode) thePOWERbrushless motor.Spindle RegisterBank of registers for controlling the various functionsMapof the spindle.WindowDetermines the duration in electrical degrees of theperiod between two consecutive zero-crosses of theBEMF in one or more phases of the spindle.Mask PWMMasks by an appropriate duration each switching orthe PSM signals generated by the block PWMconverter.Mask Win.Masks by an appropriate duration in degrees each timea tristate condition is forced for reading correctly theBEMF.BEMFBack electromotive force induced in the windings ofthe brushless motor.FrequencyFrequency multiplier.multiplierZC filterFilter for eliminating spurious switching of a zero-cross signal ZC generated at each zero-cross of theBEMF.ZC comp. + ZCIn this block the zero-cross signal is generated andmaskingall the maskings forced by the signals Mask Win andMask PWM and by the current limiter are enabled.Curr LimCurrent limiterInd S.Block for measuring inductances of the three phases ofthe used spindle, in the start-up phase, for identifyingthe position of the rotor.Async comm.This signal is completely asynchronous with the othersignals of the system.Embedded startupIntegrated automatic start-up of the spindle.DMUXBlock for selecting the output signals of the blocksinductive sense and ZC filter.SpindleBrushless motor that rotates the disk.
TABLE 2F-Fwd ADCAnalog-to-digital converter of the supply voltage level.R var.Circuit for compensating variations of resistance of theCompensat.winding of the VCM in respect to the nominal value.POR RetractCircuit that displaces the VCM in the rest position when aPower on Reset is asserted.VCM Regis-Bank of registers for controlling the various functions of theter MapVCM.IIRInfinite impulse response filter.Pwr SupplyCircuit for feed-forward compensating supply voltage shiftsFeed-Fwdfrom the nominal value.Hi PerformConverts an N bit digital datum in two phase-shiftPSM conv.modulated (PSM) signals for driving the VCM.VCM PSMPower stage for driving the VCM.POWERBEMFCircuit for compensating the back electromotive forceCompensat.induced in the winding of the VCM.Discont.Charges/discharges the VCM on/from the parking rampRamp Load.with temporaneous interruption of the current for readingthe BEMF.ADCAnalog-to-digital converter.
In the depicted example, the driving circuit of the VCM is realized according to a digital voltage mode. This requires dedicated additional control blocks for compensating unavoidable variations of the three main variables that, in this case, are not controlled, as it happens with a current feedback loop, namely: the back electromotive force induced in the winding, the supply voltage, and the resistance in the winding of the motor.
Obviously, if the control architecture of the VCM is based on a more traditional current control loop, the circuit blocks of FIG. 2, called R var. Compensat., IIR, BEMF Compensat., PWR Supply FF and F-FWD ADC would be substituted in their function by a normal feedback current control circuit (current mode driving).
These integrated devices are known as a power combo, despite the efforts for implementing control functions, safety functions for ensuring the integrity of the power stages and any other possible function in a digital mode. They require the presence of other analog circuits besides the integrated structures of the output power devices.
Constantly improved fabrication technologies for reaching larger and larger scales of integration, especially for digital circuits, have allowed a reduction in the size of devices and in their footprint. These are important factors for the manufacturers of mass storage disk devices.
Despite the new possibilities of greater compactness of the control digital circuitry offered by the new fabrication technologies, these extreme process technologies do not allow the fabrication of power devices and of related analog driving circuits of utmost performance in terms of power consumption. Further reductions of the size must be combined with the need of preserving acceptable performances of the power stages and of the related driving circuits.